1. Technical Field
The present disclosure pertains to an amplification electronic device and, more particularly, to an operational amplifier of the transconductance type that includes two amplification stages.
2. Description of the Related Art
In order to achieve a reduction of power consumption in telecommunications equipment and systems, for example, for wireless applications, there is a tendency to reduce the supply voltage of the electronic circuits that are employed in such systems. Due to the development of improved technological processes, it is now possible for digital and analog circuit systems to operate at a supply voltage of about 1.2 V.
Particularly, for applications relating to time-sampled analog networks operating at frequencies of tens to hundreds MHz, such supply voltage of the circuits imposes to implement and employ specific operational amplifiers that are capable of ensuring suitable performances for the above-mentioned applications, particularly with reference to the frequency response of such amplifiers and to the dynamics of the output signal. At the same time, such amplifiers have to ensure a low current consumption.
An operational transconductance amplifier (OTA) of known type currently employed in circuits of time-sampled analog networks, for example, at the supply voltage of 1.2 V, is the Miller amplifier. Such amplifier includes two amplification stages; therefore, it ensures a suitable voltage gain for most applications.
However, the Miller amplifier is not free from drawbacks. In fact, as it is known, the frequency response of such operational amplifier is determined by the presence of two poles corresponding to high impedance nodes of the same amplifier. Particularly, each pole is associated with a high impedance node relative to each of the amplification stages. In order to ensure the stability of such Miller amplifier, it is known to provide for a compensation capacity between the above-mentioned high impedance nodes. The introduction of such capacity allows obtaining the pole splitting effect, which is known to those skilled in the art, wherein it causes the reduction of the characteristic frequency of one of the poles, or fundamental pole, of the operational, and the increase of the second pole characteristic frequency.
In more detail, as it is known to those skilled in the art, following to compensation, the frequency response of the open loop operational amplifier is characterized by a transition frequency FT, equal to:
      F    T    =            gm      1              2      ⁢      π      ⁢                          ⁢              C                  C          ⁢                                          ⁢          1                    where CC1 is the compensation capacitance of the amplifier, and gm1 is the transconductance of the transistors of the first amplification stage. The frequency of the second pole F2 is about:
      F    2    ≅                    gm        2                    2        ⁢        π        ⁢                                  ⁢                  C          L                      ⁢          3      4      where CL is the loading capacitance of the amplifier, and gm2 is the transconductance of the transistors of the second amplification stage.
As it is known, in order to ensure the stability of the Miller amplifier, the condition has to be true that:
                                          F            T                    ≤                      3            ⁢                          F              2                                      ⇒                                            gm              2                                      gm              1                                ≥                      4            ⁢                                          C                L                                            C                                  C                  ⁢                                                                          ⁢                  1                                                                                        (                  1          ⁢          a                )            
Therefore, the transition frequency FT is limited by the second pole frequency F2.
Since the whole amplifier bandwidth is determined by the open-loop transition frequency of the response, in order to obtain a suitable band, particularly for wireless applications, it is necessary to increase the gm1 and gm2 values and, consequently, also to increase the current consumption.